Chimaeriformes
Class Chondrichthyes *Order Chimaeriformes *Family †Rhinchochimaeridae *Family †Callorhinchidae *Family Chimaeridae Chimaera's are unique holocephalid fish that usually inhabit the deep ocean. They first appeared in the early Devonian, and had unique adaptations for living in the deep sea, such as large eyes. These early species had fins on their tails, which aided in swimming, though in some species like †''Echinochimaera'', they also had long pectoral fins, used in swimming as well. These early species were slow moving and fed on hard shelled creatures. The minor extinction event at the end of the Permian had no affect on chimaera's, and instead seemed to help them continue to thrive. They became more specialized, and gained whip like tails and had their large pectoral fins become wide, and used as a primary source of locomotion. Most fossil chimaera's come from the Triassic, the most well known coming from the Namda Formation in Norway. Here the colder temperatures meant that some actually became shallow water, reef dwelling species, though they were still nocturnal. The fossil record is very complete, with hundreds of specimens found, almost 95 percent complete. The most numerous specimens were of †''Achinomera''. and †''Dalinichthys''. These shallow water species formed the Rhinochimaerids. Some species evolved long noses and snouts. Their dorsal fins lacked the venomous spikes that their previous ancestors had. However, during the Norian Hothouse Event, Rhinochimaerids were heavily affected, and ended up going extinct by 230 million years ago. With shallow water chimaera's ending in an evolutionary disaster, deep sea chimaera's thrived. Forming two groups; the callorhinchidae, and the chimaeridae. The callorhinchidae had small pectoral fins, and instead had a tail fin, which it used in swimming. They also lacked the venomous spines of other chimaera's. They only existed in the deep sea, and used tiny barbels on the snout to detect prey, mainly crustaceans. Some managed to grow to bigger sizes, like the metre long †''Zendahydrus''. However by the late early Jurassic the callorhinchidae were outcompeted by the chimaeridae and other chondrichthyes, to the point of extinction. During the Jurassic the fossil record for chimaera's substantially decreased, with fewer lagerstätte, and most isolated elements like teeth and spines. At first this was thought to be that chimaera's were going extinct, however it later turned out to be preservation bias, and that apart from the callorhinchidae, they were not going extinct. The chimaeridae were the only chimaera's left, and they became more diverse than ever before. While most still remained deep sea crustacean and bottom feeders, some became fearsome predators, informally known as "dragon chimaera's." The most well known is †''Materspinus''. This is an epipelagic genus that specializes in hunting fish. They have incredibly long, whip like tails, not used in swimming, but their pectoral fins have become even larger, and is used to swim. In addition they were very large, 3 metres long and nearly 350 kilograms. They are very fast predators, with long snouts. Some species have dorsal fins with the venomous spines, while others are devoid of them. They had competition with the agassizodontidae, similar, closely related creatures. The fossil record shows that some agassizodontidae began to hunt the dragon chimaera's, and by the early Cretaceous the dragon chimaera's became smaller and smaller until they went extinct all together. Other chimaera's evolved extreme body shapes, such as the †''Dracobatos'', stingray-like chimaeras. In some species the fins ended up being the shape of stingray fins. Other's simply had large fins, and long noses, like †''Squalimimus''. The Jurassic was the beginning of the chimaera's "Golden Age". In Cretaceous deposits the chimaera fossil record became even more incomplete, but things started to change by the end of the Cretaceous, when the number of lagerstätte remains rose tremendously, even surpassing that of Triassic rocks. However they remained very similar to the chimaeridae in our timeline. The ice age at the end of the Mesozoic actually strengthened their diversity. Many polar species evolved. In places were other predatory fish didn't evolve, primarily in arctic areas, they became predators again. Their fossils in the Cenozoic became more scarce, but there was no indication that their diversity was affected. However, their diversity was affected whenever the climate warmed up. They retreated back into the deep sea when it warmed, and when it cooled it they diversified again. Also during the Paleocene, none existed in the equatorial belt, as now they had evolved to thrive in cool conditions. They faced competition again when some xenacanthia became deep sea, but being highly adaptable they moved even deeper. One genus, †''Bivalvenator'', specialized in living almost entirely in clam beds, because they fed exclusively on clams. †''Myxinovenator'' fed mostly on hagfish. Hagfish can be hard to eat, as they produce slime that can clog up the gills of predators. However, †''Myxinovenator'' has the ability to spit the slime out and eat the hagfish, rendering the hagfish's defense mechanism useless. Another, very shark like species called †''Megapnomera'' evolved in the Miocene, and it was a deep sea scavenger, very slow moving, and harmless. However, when the current ice age began, these hunters mysteriously disappeared, for reasons we still don't understand. However, they are still common, and can be found in both the deep sea and the polar regions. In the arctic there is a shark like species called Cryochimaera. It feeds on other arctic chimaera's. There are still many undescribed genus and species of both fossil, and modern chimaera's, but it is clear that they are the most successful Chondrichthyes to have ever evolved. Category:Holocephalid Category:Chondrichthyes